Adults who sit for most of the day may have a higher risk of premature death than people who don’t sit a lot, regardless of exercise habits, according to a US study.

Being active or moving at just one specific time of the day for exercise is simply not enough, said lead study author Keith Diaz of the Center for Behavioral Cardiovascular Health at Columbia University Medical Center in New York.

Diaz said that people need to be mindful of moving frequently throughout the day in addition to exercising.

While previous research has linked excessive sedentary time to an increased risk of death, many of these studies relied on people to accurately recall and report how much they moved around and might not have painted a clear picture of the relationship between mortality and inactivity.

For the current study, researchers examined data on 7,985 adults, age 45 and older, who were asked to wear accelerometers to measure activity levels for one week.

Overall, sedentary behavior accounted for 77% of participants’ waking hours, or about 12 hours a day, researchers report in the Annals of Internal Medicine.

On average, bouts of sedentary time were about 11 minutes long, and more than half of the time people spent sitting and standing happened in bouts of less than 30 minutes, the study found.

About 14% of the people in the study typically had stretches of sedentary time lasting at least 90 minutes, however.

During the study, 340 people died after an average follow-up of four years.

Researchers sorted participants into four groups from the least sedentary people, who spent only about 11 hours total sitting and standing in a typical day, to the most sedentary people who were inactive for more than 13 hours a day.

They also sorted participants into four groups based on how long typical bouts of sedentary time lasted before people took movement breaks, ranging from less than 7.7 minutes to at least 12.4 minutes.

Compared to the least sedentary people with the briefest stretches of sitting time, the most sedentary people with the longest periods of sitting down were twice as likely to die of all cause during the study period.

One limitation of the study, however, is that the accelerometers couldn’t distinguish between sedentary time from sitting versus inactive periods when people were standing, the authors note.

The study also wasn’t a controlled experiment designed to prove how or whether sedentary time directly causes premature death.

It’s possible that prolonged sedentary stretches might hasten death by causing what’s known as metabolic toxicity, said Dr. David Alter, head of cardiovascular and metabolic research for the University Health Network-Toronto Rehabilitation Institute in Canada.

“The lack of activity in our muscles affects our ability to metabolize our sugars efficiently,” Alter, author of an accompanying editorial, said by email. “Over time, our body accumulates excess fat, which can lead to obesity, diabetes, heart disease, cancer and death.”

One thing that might help, however, is a stopwatch, because it could remind people to get up and move around often throughout the day and avoid long stretches of sedentary time, Alter said.

Even though standing desks have become increasingly popular as a possible fix to the problems caused by sedentary time, it’s not clear if they help people live longer.

Treadmill desks, under desk steppers or cycles or even just plain old-fashioned walking breaks that can be easily implemented in an office setting would be better than just sitting, Diaz suggested.